In an exhaust gas from e.g. a Diesel engine of e.g. an automobile, particulates containing carbon as the main component are contained in a substantial concentration (from 150 to 250 mg/Nm3) and thus cause an environmental problem together with nitrogen oxides, etc. Accordingly, it is desired to remove them efficiently and economically. Heretofore, various filters have been proposed to capture and remove such fine solid particles contained in exhaust gases.
For example, JP-A-57-35918 or JP-A-5-214922 discloses an exhaust gas filter wherein both ends of a honeycomb filter are alternately plugged at the upstream end or the downstream end of a plurality of channels. A honeycomb filter of this type has a structure such that a combustion exhaust gas to be cleaned, is supplied to openings on the upstream side of the filter and permitted to pass through partition walls of the filter, so that particulates in the exhaust gas are captured and removed by partition walls, and then, the exhaust gas after cleaning is taken out from openings at the downstream side of the filter.
On the other hand, the material for such a honeycomb filter is required to have not only high heat resistance but also a small thermal expansion coefficient and high thermal shock resistance, since it is exposed to a rapidly heated or cooled environment, and accordingly, silicon carbide or cordierite material has been proposed and practically used. However, such a material still has no adequate properties as an exhaust gas filter.
Namely, with an exhaust gas filter, it is likely that when captured non-combustion carbonaceous fine solid particles are abnormally deposited, such carbon will catch-fire and burn, whereby an abrupt temperature rise will take place so that the temperature locally reaches from 1,400 to 1,500° C. In such a case, a filter made of silicon carbide material will have a temperature distribution at various places of the filter, and the thermal expansion coefficient is about 4.2×10−6K−1 i.e. not so small, whereby cracks are likely to form by the thermal stress or thermal shock exerted to the material, thus leading to partial breakage. On the other hand, in the case of a filter made of cordierite material, the thermal expansion coefficient is small at a level of from 0.6 to 1.2×10−6K−1, whereby the problem of cracks due to thermal shock is less, but the melting point is not so high at a level of from 1,400 to 1,450° C., whereby a problem of partial melting due to the above-mentioned abnormal combustion of carbon becomes serious.
Once defects are formed in the interior of an exhaust gas filter by the breakage or melting of the filter as described above, the efficiency of the filter for capturing carbon decreases, and at the same time, the pressure of the exhaust gas exerted to the filter will be an excessive load to the defective portions and thus induce new breakage. Consequently, the entire exhaust gas filter will fail to function.
As the material for such a honeycomb filter, WO01/037971 proposes aluminum titanate as well as silicon carbonate or cordierite. Aluminum titanate is a material having heat resistance at a high temperature exceeding 1,700° C. and a small thermal expansion coefficient and excellent thermal shock resistance. However, on the other hand, aluminum titanate has a serious problem that since it has a decomposition region usually within a temperature range of from 800 to 1,280° C., it can not be used with stability within a fluctuated temperature region containing such a temperature range. Further, it has a difficulty such that since the anisotropy of its crystal structure is substantial, slippage by a thermal stress is likely to take place, and the mechanical strength is not high enough. Accordingly, it still has had a problem in its use for the production of a honeycomb having a thin wall thickness and a high cell density or in its use as an exhaust gas filter to be subjected to a load such as mechanical vibration at a high temperature, as mounted on an automobile or the like.
It is an object of the present invention to provide a honeycomb filter for cleaning exhaust gas, which is excellent in heat resistance, has a small thermal expansion coefficient and excellent thermal shock resistance, is free from thermal decomposition even at a high and fluctuating temperature and has high mechanical strength so that it can be used with stability for a long period of time and which is capable of capturing and removing, with high efficiency, particulates such as fine carbon particles contained in an exhaust gas from e.g. a Diesel engine, and a process for its production.